A B727 CREW OVERSHOT ASSIGNED ALT DURING DSCNT; CAUSING A TCASII TA.

Narrative

WHILE ON A VECTOR WE WERE INITIALLY CLRED TO 14000 FT MSL. DURING DSCNT; WE WERE RECLRED TO MAINTAIN 17000 FT MSL. UNABLE TO RECEIVE COS ATIS; THERE WAS A DISCUSSION ABOUT CORRECT ALTIMETER SETTING AT FL180. APCHING 17000 FT; WE RECEIVED A TFC ALERT FROM TCASII AND WENT OUTSIDE TO LOOK FOR TFC. WE THEN RECEIVED TCASII COMMAND TO CLB AND DISCONNECTED AUTOPLT AND FLEW THE TCASII UNTIL 'CLR OF TFC.' THE CREW HAD BEEN ON DUTY OVER 11 HRS AND FATIGUE WAS A CONTRIBUTING FACTOR (LONG NIGHT FLYING BACK SIDE OF CLOCK). WHEN I LOOKED BACK IN COCKPIT; I THOUGHT I REMEMBERED SEEING 16500 FT IN THE ALTIMETER. THE COPLT WAS SURE (SINCE HE WAS FLYING) THAT 'ALT SELECT' WAS SELECTED ON AUTOPLT. SINCE WE RECOVERED QUICKLY AT ABOUT 17300 FT AND RE-LEVELED AT 17000 FT MSL; WE RPTED OUR ACTION TO THE CTLR. THE OTHER ACFT ALSO RPTED RECEIVING A TCASII ALERT. FACTORS THAT MAY HAVE CONTRIBUTED: 1) FLT DUTY HAD STARTED AT XA30 LCL THE NIGHT BEFORE. RTE OF FLT WAS COS-LAS 35 MIN GND TIME; THEN LAS-OAK 2 1/2 HR GND TIE; THEN OAK-LAS 45 MIN GND TIME; THEN LAS-COS TO ARRIVE AT APPROX XL30. 2) CONFUSION AT ALTIMETER SETTING AT FL180 DIMINISHING THE EFFECT OF THE 1000 FT TO GO CALL. 3) FIRST NIGHT OF WK BEING MOST DIFFICULT NEW CREW AND TIRED. IF THE ALT WAS BUSTED; IT MAY HAVE BEEN IN THE RANGE OF 400-500 FT. IT IS POSSIBLE THE AUTOPLT WAS STILL LEVELING AT 17000 FT. THE ONLY OTHER POSSIBILITY COULD BE WRONG ALTIMETER SETTINGS.