NMAC IN A VFR CORRIDOR OF A TCA. BTWN AN SMA AND SMT ACFT.

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WE WERE AN SMA FLYING AT 4500 FT EXITING THE LOS ANGELES SPECIAL FLT RULES AREA NWBOUND. PLT CONTACTED SMO TWR AND WAS CLRED TO ENTER THE ATA VIA A DSCNT TO 2200 FT AND TO TURN EBOUND. AS WE CONTINUED A R DSNDING TURN FROM A HDG OF 312 DEGS TO 070 DEGS; PASSING THROUGH 3500 FT; A T- TAIL TWIN ENG ACFT WAS SEEN ON A COLLISION COURSE. THE PAX IN THE R SEAT SPOTTED THE ACFT AND NOTIFIED THE PLT. THE PLT IMMEDIATELY PITCHED DOWN TO PASS BENEATH THE TWIN. APPROX -0.5 G'S WAS ACFT LOAD FACTOR EXPERIENCED DURING PITCHOVER. ENG NOISE FROM THE TWIN COULD BE HEARD IN OUR CABIN AS IT PASSED OVERHEAD. CONTRIBUTING FACTORS: TWIN SEEMED TO BE ON A COURSE TO ENTER THE LOS ANGELES SFRA VIA THE SMO VOR 132 DEG RADIAL AT 3500 FT ON A SEASTERLY HDG. OUR SMA WAS EXITING THE SFRA ON THE 312 DEG RADIAL AT 4500 FT. TO AVOID THE TCA LATER ON THE RTE; OUR ACFT REQUIRED A DSCNT FROM 4500 FT; THROUGH THE VFR CRUISING ALT OF 3500 FT; TO THE EWARD ALT OF 2200 FT FINAL ALT. HEAD ON COLLISION POSSIBILITIES OCCUR ON THE VOR RADIAL WITH A DSNDING ACFT PASSING THROUGH OTHER VFR CRUISING ALTS. IT IS NOT KNOWN WHETHER OUR ACFT WAS ON THE VOR RADIAL OR OFFSET TO THE E. IT IS ALSO NOT ABSOLUTELY KNOWN WHETHER THE NEAR MISS OCCURRED AT EXACTLY 3500 FT. LOW TIME PLT. PLT WAS VISUALLY CONFIRMING LANDMARKS FOR PILOTAGE NAV. THIS NEEDS TO BE COMBINED WITH HORIZON SCAN IN CONGESTED AIRSPACE. EXPERIENCE HELPS. ALTERNATE RTE NWWARD THROUGH THE TCA VIA THE LA SHORELINE RTE DENIED TO PLT BY LOS ANGELES APCH CTLR DUE TO TFC CONGESTION. THIS CLRNC WOULD HAVE HELPED US AVOID LARGE CLBS OR DSCNTS FROM OUR INITIAL 2500 FT ENRTE ALT. CORRECTIVE ACTIONS: PITCH DOWN TO AVOID APCHING ACFT. DIVERSION TO R TOO SLOW AND NOT PRACTICAL AT THAT INSTANT. 2-3 SECONDS ELAPSED BTWN CTL INPUT AND ACFT PASSING OVERHEAD. APPROX -0.5 G LOAD FACTOR WAS EXPERIENCED. PERCEPTIONS; JUDGEMENTS; DECISIONS: PLT WAS DSNDING AND SCANNING LANDSCAPE FOR PILOTAGE/NAV. ATTN DIVERTED AND ACFT SCAN PATTERN WAS ALTERED. AWARENESS OF ACFT TFC RTES WAS NOT CONSTANTLY KEPT IN MIND. UPON VISUAL CONTACT WITH TFC; QUICK DECISION WAS MADE TO DSND SINCE IT IS FASTER TO DSND THAN TO CLB OR TURN R. CONCERN WAS IF OTHER ACFT ALSO DECIDED TO DSND FOR COLLISION AVOIDANCE. OTHER ACFT TOOK NO ACTION AND CONTINUED STRAIGHT AND LEVEL.