AN EMB120 FLC HAS A POTENTIAL CONFLICT WITH A C182 AFTER CANCELING IFR WITH SCT FOR A DIRECT TO THE ARPT E OF CRQ; CA.

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WE WERE AT 9000 FT MSL; ENRTE FROM LAX TO CRQ. APPROX 20 NM W OF OCN; WE WERE GIVEN A XING RESTR TO BE AT 5000 FT MSL 15 NM W OF OCN; WHICH NECESSITATED A RAPID DSCNT. WHILE IN THE DSCNT; SOCAL DIRECTED US TO SLOW FROM 250 KIAS TO 150 KIAS. USUALLY; WE ONLY ACCEPT A SPD RESTR THIS SLOW ON BASE OR FINAL. SOCAL THEN SWITCHED US TO THE NEXT CTLR; WHO FURTHER RESTR US TO 140 KIAS; AND SAID WE WERE #5 FOR THE FIELD. AT THIS POINT; THE CAPT STATED; 'WE HAVE THE FIELD IN SIGHT; CANCEL IFR; WE'RE PROCEEDING DIRECT TO THE ARPT.' THE CTLR ACKNOWLEDGED WITH 'ROGER; PROCEED TO THE ARPT.' THE CAPT THEN SAID 'LEAVING 5000 FT;' AND THE CTLR AGAIN SAID 'ROGER.' SHORTLY AFTER THAT; THE CTLR TOLD US TO FLY HDG 140 DEGS. JUST AFTER TURNING TO 140 DEGS; I SPOTTED A HIGH WING SEL CESSNA AT 10 O'CLOCK POS; AND I CALLED IT OUT TO THE CAPT; ADVISING HIM TO COME R. AFTER TAKING ACTION; HE ADVISED ATC THEIR VECTOR HAD CREATED A TFC CONFLICT; AND ASKED IF THE CTLR HAD HEARD US CANCEL. THE CTLR SAID HE HAD NOT; AND GAVE US A LANDLINE NUMBER. ON THE GND; THE CAPT TALKED TO A SUPVR AT THAT NUMBER; WHO SAID 'BOTH' CTLRS DID NOT HEAR US CANCEL IFR. LESSONS LEARNED: WE ASSUMED THE CTLR'S CALL TO PROCEED DIRECT; AND HIS ACKNOWLEDGEMENT WE WERE LEAVING OUR ASSIGNED ALT CONSTITUTED ACKNOWLEDGEMENT WE HAD CANCELED IFR. IN THE FUTURE; WE'LL GET A CLR ACKNOWLEDGEMENT; SUCH AS 'SQUAWK VFR' OR 'CANCELLATION RECEIVED;' EVEN WHEN THE RADIOS ARE BUSY. FORTUNATELY; OUR VIGILANCE IN SCANNING FOR TFC PAID OFF; AND WE WILL CONTINUE TO BE VIGILANT VFR OR IFR; ESPECIALLY IN HIGH CTLR WORKLOAD SITS. FINALLY; WE WON'T HESITATE TO SAY 'UNABLE' TO UNREASONABLE SPD RESTRS. 150 KIAS IS VERY CLOSE TO OUR MINIMUM MANEUVERING SPD WITH 15 DEG FLAPS.