ALT BUST.

Narrative

DURING THE DSCNT PHASE; WE WERE GIVEN CROSS SWEET INTXN AT AND MAINTAIN 9000 FT. THE FO; WHO WAS FLYING; THOUGHT HE HEARD 7000 FT; SINCE THAT IS WHAT HE DEPICTED ON THE WILLIAMSPORT 1 (FQM.FQM1) ARR. AS THE FO APCHED 9000 FT; IT WAS CLR HE WAS NOT LEVELING OFF; SO I IMMEDIATELY QUESTIONED HIS UNDERSTANDING OF THE ATC ALT ASSIGNMENT. HE INSISTED 7000 FT; I IMMEDIATELY QUESTIONED OUR ATC LAST ASSIGNED ALT WITH ZNY AND 9000 FT WAS CONFIRMED. THE ACFT DSNDED BELOW 9000 FT BY 250 FT; PLUS OR MINUS 50 FT. I INFORMED ATC WE WOULD IMMEDIATELY CLB BACK TO 9000 FT BUT ATC OFFERED 8000 FT SO WE COULD CONTINUE A NORMAL DSCNT PROFILE. WE CONTINUED TO EWR ARPT OUR FLT PLAN DEST; WITHOUT FURTHER INCIDENT. THERE WAS NO TFC IN OUR IMMEDIATE VICINITY DURING OUR 250 FT ERROR AS CONFIRMED BY OUR ONBOARD TCASII. I'M NOT SURE WHY WE WERE GIVEN 9000 FT VERSUS 7000 FT AS DEPICTED ON THE WILLIAMSPORT 1 ARR AS THERE WAS NO CONFLICTING TFC IN OUR AREA. PLT FATIGUE DEFINITELY PLAYED A ROLL IN THIS AS WE HAD BEEN FLYING ALL NIGHT LONG FROM HOUSTON; TX; VIA LAS VEGAS. SUPPLEMENTAL INFO FROM ACN 292440: I THOUGHT I HEARD 7000 FT AND SET 7000 FT IN ALT WINDOW -- SINCE THAT IS WHAT IS DEPICTED ON THE WILLIAMSPORT 1 (FQM.FQM1) ARR.