COMMUTER FLC OF EMB120 HAS NMAC WITH BE-136 ON DSCNT.

Narrative

WE WERE CLRED TO DSND FROM 10000 FT TO 8000 FT. (ENRTE FROM PSP TO LAX.) PASSING THROUGH 9700 FT; FO (PNF) SAW A TCASII TARGET POP-UP. FO SELECTED 5 MI RANGE ON TCASII AND TARGET DISAPPEARED; THEN CAME BACK INDICATING 12 O'CLOCK AND 300 FT BELOW US AT 3 MI. I DISENGAGED THE AUTOPLT AND SLOWED OUR RATE OF DSCNT WHILE SCANNING FOR THE TFC. WHEN I COULD NOT ACQUIRE THE TFC VISUALLY AND THE TCASII SHOWED IT AT 12 O'CLOCK; LESS THAN 1 MI; 100 FT BELOW I PITCHED THE ACFT NOSE UP; INITIATING A CLB. AT THIS POINT I SAW THE TFC PASSING BELOW; APPROX 200 FT OFF THE L WINGTIP. ATC HAD ACQUIRED THE TARGET AT VIRTUALLY THE SAME TIME AS WE DID. THE CTLR SPECULATED THAT THE OTHER PLT SAW US AND THEN TURNED ON HIS XPONDER. IF THAT IS THE CASE THEN THIS NEAR MISS MIGHT HAVE BEEN AVOIDED HAD THE PLT TURNED ON HIS XPONDER ON TKOF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT IN A CONVERSATION WITH A SUPVR AT APCH HE LEARNED THAT ATC WAS TRACKING THE ACFT BUT LOST HIM WHEN HE WENT THROUGH THE BANNING PASS. THERE HAS BEEN NO FURTHER ACTION. RPTR WAS ON V388 AIRWAY AND ON AN IFR FLT PLAN.