B737 ARR TO LAS DSNDS BELOW ASSIGNED ALT CONFLICTING WITH TFC.

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WE WERE CLRED TO CROSS LUXOR INTXN ON THE GRNPA ONE RNAV ARR INTO LAS AT 12000 FT; EXPECTING AND BRIEFED FOR A VISUAL APCH AND LNDG ON RWY 25R. PRIOR TO REACHING 12000 FT AND OR LUXOR; WE WERE RECLRED TO PROCEED DIRECT TO THE LAS VOR; MAINTAIN 12000 FT; EXPECT VISUAL RWY 1L. WHILE PROGRAMMING THE FMC AND RE-BRIEFING THE APCH TO THE NEW RWY; WE WERE AGAIN GIVEN A NEW CLRNC FOR A TURN OFF THE DIRECT COURSE TO THE VOR; A DSCNT; AND A SPD RESTR. THE NEW COURSE; SPD AND AN ALT OF 5000 FT WERE SET IN THE MCP WINDOW AND CONFIRMED BY BOTH CREW MEMBERS. DSNDING THROUGH 8000 FT; THE CTLR ISSUED A TA TO WHICH THE PNF (CAPT) RESPONDED 'LOOKING.' DSNDING THROUGH 7800 FT ALMOST SIMULTANEOUSLY WE RECEIVED A TCASII RA TO 'REDUCE DSCNT' AND THE CTLR TOLD US TO CLB TO 8000 FT FOR TFC. WE IMMEDIATELY COMPLIED WITH BOTH COMMANDS AND CLBED FROM 7700 FT BACK UP TO 8000 FT. THE CAPT THEN QUERIED THE CTLR ABOUT OUR ALT CLRNC; WHICH BOTH PLTS UNDERSTOOD TO BE 5000 FT. THE CTLR STATED THAT WE WERE CLRED TO 8000 FT; AND THEN GAVE US ANOTHER HEADING AND FREQ CHANGE. UPON ARR AT LAS; WE WERE INSTRUCTED TO PHONE THE LAS TRACON FOR A POSSIBLE PLTDEV. THE TRACON MGR INFORMED US THAT THE TAPES HAD ALREADY BEEN REVIEWED. HE STATED THAT HE HEARD THE CLRNC AS 'TURN 5 DEGS L; DSND TO 9000 FT; SLOW TO 210 KTS' AND HEARD THE CAPT REPEAT THE CLRNC TO 9000 FT. HE FURTHER POINTED OUT THAT THE CLRNC APPEARED TO HAVE BEEN INCORRECTLY STATED AS 'NINE' THOUSAND RATHER THAN 'NINE-ER' THOUSAND. CONTRIBUTING FACTORS TO THIS INCIDENT INCLUDED MULTIPLE CLRNCS/VECTORS OFF THE CHARTED ARR PROC; HIGH WORKLOAD REPROGRAMMING AND RE-BRIEFING A NEW APCH CLOSE TO THE ARPT; AND MULTIPLE INSTRUCTIONS IN ONE CLRNC BY THE CTLR; POSSIBLY INCLUDING A CLRNC USING INCORRECT PHRASEOLOGY. THE ACFT WAS PROGRAMMED WITH THE ALT BOTH CREW MEMBERS THOUGHT THEY HAD HEARD IN THE CLRNC. IF THE CLRNC TO 9000 FT WAS INDEED READ BACK CORRECTLY; I CAN ONLY ASSUME THAT THE HIGH WORKLOAD CREATED IN COMPLYING WITH SEVERAL MULTI-PART CLRNCS CAUSED THE CONFUSION THAT RESULTED IN US REMEMBERING; SETTING AND CONFIRMING THE INCORRECT ALT OF 5000 FT IN THE MCP PANEL. FURTHER CONSIDERATIONS FOR THE CREW INCLUDED THE FACT THAT THERE WERE HIGH WINDS AT LAS (UP TO 31 KT XWIND GUSTS) AND SUBSEQUENT ATYPICAL RWY IN USE. THE AIRLINE HAD ALSO RECENTLY CHANGED TO A NEW TERMINAL/GATE AT THE ARPT WITH UNFAMILIAR RWY EXIT AND TAXI CONSIDERATIONS ALL OF WHICH NEEDED TO BE BRIEFED PRIOR TO LNDG. IT WAS A CLR DAY WITH UNRESTR VISIBILITY; AND SINCE THE NEW RWY HAD NO VERT OR HORIZ GUIDANCE; WE HAD PROGRAMMED THE GLIDE PATH INTO THE FMC THAT SHOWED AN ALT OF 5200 FT AT 10 MI FROM THE RWY. FOR THIS REASON; THE CLRNC TO 5000 FT SEEMED REASONABLE. WE WONDERED ABOUT THE FACT THAT WHEN THE CTLR ISSUED THE TFC ALERT; HE SAID NOTHING ABOUT OUR ALT; WHICH WAS ALREADY A THOUSAND FT BELOW THE SUPPOSED ASSIGNED ALT OF 9000 FT AND CONTINUING DSCNT. WHEN QUERIED; THE CTLR SEEMED CONFUSED ABOUT THE ASSIGNED ALT AS WELL. THE ERROR WAS DISCOVERED ONLY BECAUSE OF THE TCASII RA AND LOSS OF VERT SEPARATION WITH THE TFC. CORRECTIONS WERE TO COMPLY IMMEDIATELY WITH BOTH TCASII AND CTLR INSTRUCTIONS; REDUCING DSCNT RATE AND THEN CLBING TO 8000 FT. IDEAS FOR PREVENTING RECURRENCE AND OR CORRECTING THE SIT INCLUDE: CTLRS NEED TO LIMIT THE NUMBER OF INSTRUCTIONS GIVEN IN 1 CLRNC; AND ALSO THE NUMBER OF 'RE-CLRNCS' THAT INCREASE PLT WORKLOAD; ESPECIALLY DURING MORE CRITICAL POINTS IN THE DSCNT AND APCH (OR TKOF AND CLB) PHASE OF THE FLT. USE OF PROPER PHRASEOLOGY -- 'NINE-ER' VERSUS 'NINE' ALSO REDUCES POTENTIAL FOR CONFUSION. BY ADHERING TO THE CHARTED PROCS; ALT AND SPD RESTRS CAN BE ANTICIPATED; BRIEFED; AND PROGRAMMED INTO FMC'S WHEN AVAILABLE; THEREBY REDUCING PLT AND CTLR WORKLOAD AND ALSO GIVING PLTS A 'GAUGE' TO DETERMINE IF A CLRNC SEEMS UNREASONABLE OR INCORRECT. CLRNCS OFF OF CHARTED RTES; ALTHOUGH FAIRLY STANDARD PROC; INCREASE THE POSSIBILITY OF SPD AND ALTDEVS AND ALSO THE RISK OF INADEQUATE TERRAIN CLRNC IN MOUNTAINOUS TERRAIN WHEN IMC. OUR CURRENT COMPANY PROC REQUIRES THE PF TO SET ALL MCP CHANGES WHEN THE AUTOPLT IS ENGAGED; BUT SEVERAL OTHER COMPANIES HAVE THE PNF SET THE ALT IN THE ALERTER AT THE TIME HE OR SHE IS ACCEPTING THE CLRNC. SINCE THE PF ALREADY HAS TO SET HEADING; SPD; AND MCP MODE AND MONITOR THE ACFT; THIS PROVIDES ONE LESS CRITICAL ITEM/DUTY FOR THE PF AND THEREFORE ONE LESS POSSIBILITY OF ERROR. LASTLY; THROUGH ANOTHER SOURCE; WE FOUND OUT THAT THE LAS TRACON HAD AT LEAST 9 VIOLATIONS GENERATED THE SAME DAY. WHILE WE HAVE NO SPECIFIC DETAILS ABOUT THE VIOLATIONS; AND THEREFORE CANNOT SPECULATE ON THE CAUSES; THAT SEEMED LIKE AN ABNORMALLY HIGH NUMBER.