B737 CREW FAILS TO HOLD SHORT OF DESIGNATED TXWY AS INSTRUCTED; AFTER LNDG AT LAS.

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LANDED RWY 25L AT LAS EXITED AT TXWY A6. WE WERE INSTRUCTED TO TURN R; TAXI DOWN TXWY A TO HOLD SHORT OF TXWY A5 ON TXWY A INSTEAD I CONTINUED TO TAXI ONTO TXWY A5 (L TURN) AND HELD SHORT OF RWY 25R. ATC REQUESTED FOR US TO CALL TWR (AFTER CLRNC TO CROSS RWY 25R) AT THE GATE. CALLED AND SUPVR INDICATED THAT THE READBACK OF INSTRUCTIONS WAS CORRECT BUT THE INSTRUCTIONS WERE NOT FOLLOWED. HE WANTED ME TO HOLD SHORT OF TXWY A5 SO THAT COMPANY LNDG BEHIND US COULD EXIT AT TXWY A5 AND CLR RWY 25L. THERE WASN'T A CONFLICT WITH COMPANY EXITING TXWY A5 AND ANY OTHER LNDG TFC BEHIND THEM. BUT THE POTENTIAL WAS THERE. IT JUST DIDN'T PRESENT ITSELF THAT DAY. ATC CROSSED BOTH OF US ACROSS RWY 25R. AS CAPT; READ BACK THE INSTRUCTIONS (WHICH I DID) BUT I CAN'T REMEMBER IF I READ IT BACK CORRECTLY TO THE FO. IF I HAD VERBALLY SAID TO THE FO AS I APCHED TXWY A5 THAT I WAS GOING TO TURN L AND HOLD SHORT OF RWY 25R THAT MIGHT HAVE SAVED ME FROM MY MISTAKE. I SHOULD HAVE INDICATED MY TAXI INTENTIONS TO THE FO BEFORE MAKING THE TURN.