NMAC BTWN AN LTT ACR AND A PVT SMA SEL ACFT ON BUSY AIRWAY DURING DSCNT OF THE LTT RPTING ACFT AND CLB OF THE OTHER SMA ACFT.

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THIS LEG WAS MY TURN TO FLY. AFTER DEPARTING PSP WE LEVELED OFF AT 12000 FT MSL ON V388 WBOUND; CTLED BY ONT APCH CTL ON FREQ 119.65. AT A CERTAIN POINT ONT ISSUED A CLRNC TO DSND TO CROSS PDZ VOR AT 8000 FT TO WHICH WE READ BACK AND EXECUTED. AT APPROX 20 NM E OF PDZ AS WE WERE DSNDING THROUGH 10000 FT; ONT ISSUED TFC 11 O'CLOCK; 3 MI; 9300 FT CLBING OPPOSITE DIRECTION; TYPE UNKNOWN. WE RESPONDED; LOOKING; AND ASKED TO STOP DSCNT IMMEDIATELY. SO WE LEVELED OFF AT 9600 FT AND STAYED ON THE AIRWAY. I WOULD LIKE TO ADD THAT MY ACFT WAS EQUIPPED WITH AN EXPERIMENTAL TCASII MADE BY BF GOODRICH. THE TCASII WAS DEPICTING THE TFC BUT DUE TO THE TOPS OF THE CUMULUS CLOUDS BEING RAGGED BTWN 8500 FT AND 9500 FT; WE COULDN'T SEE THE TFC. ONT CONTINUED TO CTL OTHER TFC AND A FEW SECONDS LATER ISSUED US FREQ CHANGE TO 135.40 BUT REVERSED THE CLRNC AND ISSUED A TA 12 O'CLOCK LESS THAN A MI TO WHICH WE RESPONDED AND LOOKING. 5 SECONDS LATER THAT TFC PASSED AT 2:30 - 3 O'CLOCK; LESS THAN 1/8 OF A MI. THE TFC WAS AT THE SAME ALT; A MARCHETTI SF260 PAINTED MIL OLIVE GREEN WITH BRIGHT ORANGE WINGTIPS AND SPINNER. ONLY THEN DID THE TCASII ISSUE A VOICE ADVISORY 'TFC; TFC.' I SHOULD MENTION THAT SOON AFTER ONT ISSUED THE TA THE CTLR ISSUED THE FREQ CHANGE AGAIN AND CONTINUED TO CTL OTHER TFC WITHOUT HIM OR US IDENTING THIS TFC; AND WE COULDN'T XMIT BECAUSE OF FREQ CONGESTION. EVENTUALLY WE SWITCHED AND PROCEEDED NORMALLY.