ACR HAS NMAC WITH SMA ON FINAL APCH INSIDE TCA. TCASII RA.

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CONDITIONS WERE VFR AND WE HAD BEEN CLRED FOR THE SAN LOC RWY 27 APCH. OUR POS WAS INSIDE VYDDA. THE CHART ALLOWS A STEP DOWN TO 3600 FT INSIDE VYDDA; BUT SINCE THE ARPT WAS IN SIGHT I WAS NOT HURRYING TO REACH THE MIN ALLOWABLE ALTS PRESCRIBED BY THE APCH. OUR ALT WAS ABOUT 4200 FT; ON THE LOC WHEN APCH GAVE US TFC AT 12 O'CLOCK; RADAR ALT OF 3600 FT. I LOOKED UP JUST IN TIME TO SEE A LIGHT PLANE TRAVEL FROM L TO R DIRECTLY IN FRONT OF OUR ACFT. WE GOT A TA FROM THE TCASII ABOUT THE TIME THE CTLR ALERTED US TO THE TFC. I WOULD ESTIMATE THAT WE CAME AS CLOSE AS 500 FT TO THE OTHER ACFT. I ASKED ATC IF THE PLANE WAS UNDER HIS CTL AND HE SAID THAT IT WAS NOT AND THAT WAS THE FIRST TIME HE HAD SEEN HIM. NO MANEUVERING WAS NECESSARY TO AVOID THE OTHER ACFT. HAD THE WX CONDITIONS BEEN SLIGHTLY WORSE I WOULD HAVE BEEN MORE CONCERNED WITH REACHING THE PRESCRIBED APCH ALTS SOONER. THERE IS LITTLE DOUBT IN MY MIND THAT IF WE HAD BEEN AT THE STEP DOWN ALT OF 3600 FT A CATASTROPHE WOULD HAVE BEEN NEARLY UNAVOIDABLE. I SPOKE WITH ATC ON THE TELEPHONE AFTER LNDG IN SAN. HE SAID THEY TRACKED THE AIRPLANE NBOUND UNTIL THEY LOST IT ON RADAR. NO ONE TO HIS KNOWLEDGE WAS CTLING THE ACFT. HE SAID HE WOULD ATTEMPT TO TRACE WHERE THE ACFT HAD COME FROM BUT EXPRESSED LITTLE HOPE IN IDENTING THE PLT. I DO NOT KNOW IF THIS SITUATION IS TECHNICALLY A NEAR MISS. WHAT CONCERNS ME THE MOST WAS THE POTENTIAL FOR A DISASTER HAD WE BEEN AT THE LOWEST PRESCRIBED ALT OF 3600 FT.