B737-300 FLT CREW AT LAS RECEIVES AND EXECUTES A TAXI CLEARANCE FROM RAMP CONTROL THAT IS IN DIRECT CONFLICT WITH A B757 WHO IS COORDINATING WITH GROUND CONTROL.

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UPON LNDG IN LAS ON RWY 25L; TWR CLRED US TO CROSS RWY 25 AT INTXN A5 WITHOUT DELAY AND CONTACT LAS RAMP CTL DIRECTLY. WE CROSSED RWY 25R AT INTXN A5 ONTO TXWY B. DUE TO THE CONGESTED XMISSION ON RAMP CTL; WE CAME TO A FULL STOP FACING N TOWARDS TERMINAL C ALLOWING DISTANCE FOR A TURN L OR R ON TXWY B AWAITING DIRECTIONAL CLRNC FROM RAMP CTL. WE CALLED RAMP CTL WITH OUR GATE ASSIGNMENT. RAMP THEN CLRED US TO TRANSITION TO TXWY C AS QUICKLY AS WE CAN. I NOTED ANOTHER CARRIER'S B757 APCHING SPOT Y OR SPOT X FROM THE RAMP RATHER QUICKLY AND INTXN B5 WOULD BE HOLDING HIM UP FOR HIS TAXI ONTO TXWY C. I HESITATED MOMENTARILY BEFORE COMMITTING MY DIRECTION OF TRAVEL KNOWING INTXN B4 WOULD BE A BETTER WAY FOR US AND THE OTHER CARRIER. RAMP THEN SAID; 'IF YOU CAN STILL MAKE THE TURN FOR INTXN B4 TO THE GATE THAT WOULD BE GREAT.' I TURNED THE ACFT R (E) ON TXWY B AND PROCEEDED TO INTXN B4. WHILE E ON TXWY B; I COULD NOT SEE THE OTHER CARRIER. APCHING INTXN B4; MY VIEW OF TXWY C SHOWED CLR OF TFC. I THOUGHT THEY WERE STILL BACK AND PROBABLY HOLDING FOR US OR HAD TRANSITIONED BEHIND US FOR TXWY B. MY FO ASKED AS I STARTED THE L TURN AT INTXN B4; 'WHERE IS THE OTHER CARRIER?' I TOOK ANOTHER HARD LOOK AS I BEGAN THE TURN AND THEY CAME INTO VIEW TRAVELING RAPIDLY EBOUND RIGHT WHERE I WOULD BE TURNING IN FRONT OF HIM. I COULD SEE THAT HE COULD NOT STOP IN TIME FOR ME. I STOPPED ABRUPTLY AND WATCHED IN DISBELIEF AS HE SWIFTLY PASSED. I COMPLAINED TO RAMP CTL INFORMING HIM OF THE CLOSE ENCOUNTER. HE THOUGHT THE COORD HAD BEEN MADE WITH GND CTL. WE CONTINUED TO THE GATE WITHOUT FURTHER INCIDENT. AT FIRST WE THOUGHT THE OTHER CARRIER HAD TRIED TO BEAT OUR TAXI IN ORDER TO CROSS BEFORE US WITH TOTAL DISREGARD OF SAFETY. BUT IT BECAME CLR THAT WE HAVE NO IDEA WHAT HE HAD PREVIOUSLY BEEN CLRED TO DO. HE WAS TALKING TO GND CTL AND PROBABLY THOUGHT WE WERE STOPPING FOR HIM. WE WERE BOTH FOLLOWING CLRNCS TO CROSS THE SAME PIECE OF CONCRETE AT THE SAME TIME BY 2 DIFFERENT CTLING AGENCIES. RAMP CTL SEEMS TO HAVE TOO MUCH AUTH THAT GOES BEYOND RAMP CTL. WHY ARE WE BEING CTLED BY RAMP CTL ON THE SAME TXWY THAT A GND CTLR IS CTLING? PLTS NEED TO HEAR WHAT OTHER PLTS ARE BEING CLRED TO. THIS HELPS US ALL BACK EACH OTHER UP TO AVOID THESE HAZARDS.