A B737-300 PLT RPTS A GAR AT LAS DUE TO TASK SATURATION. THE ALTIMETER WAS MISSET DURING APCH AND GAR BECAUSE OF OLD ATIS INFO WHICH UPSET ATC.

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SUBSEQUENT TO A GAR LAS RWY 19L; ATC ASSIGNED 6100 FT AND HANDED US OFF TO FREQ 135.0. THE NEW CTLR TOLD US TO MAINTAIN 6100 FT AND WE ADVISED THAT WE WERE AT 6100 FT. HE SAID WE WERE SHOWING 5900 FT AND IMPATIENTLY TOLD US TO MAINTAIN 6100 FT STATING AN ALTIMETER SETTING OF 30.07. WE INFORMED HIM THAT WE WERE USING 30.27 AND WOULD MAKE THE CORRECTION. ATC REITERATED THE ALTIMETER SETTING OF 30.07 AND TOLD US AGAIN; UNNECESSARILY; TO MAINTAIN 6100 FT. HIS 'IRRITATION' WAS PARTLY JUSTIFIED AS WE HAD ASKED FOR SEVERAL INSTRUCTIONS TO BE REPEATED SINCE WE INITIATED THE GAR DUE TO TASK SATURATION. WITHIN A FEW MOMENTS OF INITIATING THE GAR (WHILE CONFIGURING THE AIRPLANE) WE WERE ISSUED 2 ALTS; A HDG; DIRECT TO A FIX NOT ON OUR CURRENT RTE; A FREQ CHANGE; AND A CHANGE OF RWY TO EXPECT. WE SOON REALIZED WE HAD INADVERTENTLY SET THE ALTIMETER SETTING USING ATIS INFO FROM THE PREVIOUS STATION. UNFORTUNATELY; I FAILED TO EXERCISE MY PERSONAL RULE OF ALWAYS X'ING OUT THE BACK SIDE OF THE ATIS NAPKIN TO PREVENT THIS VERY ERROR. I PRESUME WE WERE 200 FT LOW ON ALL THE XINGS ON THE GRNPA ARR TO LAS; ALTHOUGH ATC NEVER MENTIONED IT. THE FO DID AN EXCELLENT JOB OF SMOOTHLY FLYING THE GAR AND MAINTAINING GOOD ACFT CTL AS I WORKED THROUGH THESE DIFFICULTIES WITH ATC. WE COULD HAVE AVOIDED THE INITIAL GAR BY ADJUSTING MORE RAPIDLY TO A CLRNC TO RWY 19L WHEN WE WERE EXPECTING RWY 25L OR BY RECOGNIZING THE NEED FOR AN S-TURN TO THE N BEFORE JOINING THE VERY SHORT FINAL TO RWY 19L COMING OFF THE GRNPA ARR.