AN ACR MLG CLBED BACK TO ITS ASSIGNED ALT IN RESPONSE TO A TCASII COMMAND.

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ON VISUAL APCH TO LAS RWY 25L. WE WERE ON A L DOWNWIND CLRED FOR THE VISUAL; FOLLOW MLG Y JET AT THE STADIUM; REMAIN AT 6000 FT UNTIL TURNING FINAL AT APPROX 6 MI FROM RWY. I SAW AN SMT PARALLELING OUR COURSE AND CONVERGING AT WHAT APPEARED TO BE OUR ALT OR VERY CLOSE TO OUR ALT. TO MY RECOLLECTION; THIS TFC WAS NOT SPECIFICALLY BROUGHT TO OUR ATTN BY APCH; I.E.; POS; ALT; ETC. WHEN I SAW THIS ACFT; MY INSTINCTS WERE TO BEGIN A DSCNT; THEN TURN BASE. WHILE ON BASE WE RECEIVED AN RA ON TCASII COMMANDING A FURTHER DSCNT; WHICH WE COMPLIED WITH. WE WERE TOLD BY APCH THAT WE WERE REQUIRED TO REMAIN AT 6000 FT UNTIL FINAL AND PLEASE CALL WHEN WE WERE ON THE GND. AS IT TURNED OUT; THE TFC WAS AIR TAXI RTED EBOUND AT 5500 FT. IF I HAD REMAINED AT 6000 FT THERE WOULD HAVE BEEN NO PROB. HOWEVER; IF I HAD KNOWN ABOUT THE TFC; ALT; ETC; MY REACTIONS WOULD HAVE BEEN DIFFERENT. THE UNKNOWN ASPECT OF THIS TFC CAUSED ME TO TAKE THE ACTIONS THAT I TOOK. I HAVE NOT FLOWN INTO LAS IN APPROX 5 YRS AND WAS UNFAMILIAR WITH THEIR SYS OF ROUTING AIR TAXI. I FEEL THAT IF I HAD KNOWN ABOUT THIS TFC; PARTICULARLY HIS ALT; MY REACTIONS WOULD HAVE BEEN DIFFERENT. SUPPLEMENTAL INFORMATION FROM ACN 253049: IN A SUBSEQUENT PHONE CALL MADE TO APCH; WE WERE TOLD THAT HE WAS IN A CORRIDOR OR ON A ROUTING TO THE CANYON. HE ALSO TOLD US THIS HAS BEEN A PROB.