B737 FLT CREW INADVERTENTLY TURN ONTO NEW; NOT YET OFFICIAL TXWY Z; INSTEAD OF CLRED TXWY G.

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LANDED ON RWY 19L; EXITED TXWY W. GND TOLD US TO TAXI TO GATE VIA A L TURN ONTO TXWYS D TO G TO THE RAMP AND TO CONTACT RAMP CTL. AS WE PROGRESSED NBOUND ON TXWY D; WE MADE A R TURN ONTO A TXWY (WE THOUGHT IT WAS TXWY G) BUT IT TURNED OUT TO BE TXWY Z. WE TOLD GND THAT WE HAD MISSED THE TURN ONTO TXWY G AND IF WE SHOULD MAKE A 180 DEGS AND BACK TRACK? THE CTLR REPLIED 'NO PROB; TAXI VIA TXWYS Z TO W; CONTACT RAMP CTL UPON JOINING TXWY W.' I CALLED RAMP AND SAID THAT WE WERE INBOUND. RAMP TOLD US TO STOP WHERE WE WERE AND THAT WE HAD CUT OFF AN ACFT NBOUND ON TXWY G. WE HAD STOPPED THE ACFT IN THE INTXN OF TXWYS W AND G. THE RAMP CTLR ASKED US FOR 'OUR NUMBERS;' WHICH BOTH OF US REALLY DID NOT UNDERSTAND WHAT HE WANTED. AFTER 10 SECONDS OR SO; HE CLARIFIED THAT HE WANTED TO KNOW WHAT OUR GATE WAS; AND TOLD US TO TAXI TO THE GATE. THERE HAD BEEN A DC9 TAXIING N ON TXWY G BTWN TXWYS B AND W AND WAS NOT A FACTOR; AT LEAST 1000 FT AWAY (AS WE CAME TO A STOP HE HAD DIMMED HIS TAXI LIGHT). ALSO; THERE WAS ANOTHER CARRIER HOOKED UP TO A TUG NEAR TAXI SPOT X WHICH DID NOT SEEM TO BE MOVING. FACTORS: 1) GND-RAMP COORD. BASED ON A DIFFERENT TAXI RTE THAN WAS ORIGINALLY ISSUED; WE SHOULD HAVE BEEN ISSUED ALTERNATE INSTRUCTIONS TO EITHER HOLD SHORT OF TXWY W ON TXWY Z AND CALL THE RAMP OR; TO MAKE THE TURN AT TXWY W AND TO HOLD SHORT OF TXWY D ON TXWY W AND CALL RAMP. 2) FAMILIARITY. THE USUAL TAXI FLOW FROM RWY 19L IS VIA TXWY W; ALTHOUGH THE ISSUED RTE WAS NOT THAT DIFFICULT TO FOLLOW; THE TXWY WE TURNED ONTO (TXWY Z) IS NOT DEPICTED ON THE DEC/XA/03; 10-9 FOR LAS (IT PARALLELS TXWY G JUST TO THE W AND HAS TXWY LIGHTING AND TAXI MARKS; BUT DID NOT HAVE A SIGN OR PAINT NEAR WHERE WE MADE THE TURN). AT THE END OF 5 LONG LEGS AND FACING AN ACFT SWAP WE DID NOT CATCH THE ERROR UNTIL WE HAD ALREADY MADE THE TURN. 3) IF TXWY Z WAS NOT 'OFFICIALLY' OPEN YET; IT SHOULD HAVE HAD SOME SORT OF LIGHTING; BARRICADES OR OTHER MARKINGS THAT WOULD HAVE CUED US INTO SEEING THAT IT WAS NOT OUR TXWY.