NMAC BETWEEN ACR-SMT AND GA-SMA THAT MADE UNAUTH PENETRATION OF ATA.

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WE; FLT NUMBER XXA; WERE CLRED FOR A VISUAL APCH TO RWY 34R OAT SMF PRIOR TO REACHING SAC VOR; AT WHICH TIME SAC APCH BEGAN WORKING ANOTHER COMPANY ACFT WITH FLT XXB SEQUENCING HIM BEHIND US. OTHER ACFT INVOLVED WAS AN MLG FOR THE LEFT RWY AT OUR 8 O'CLOCK (NO FACTOR) AND AN SMA Y; ALSO BEING SEQUENCED FOR AN APCH TO SMF. THE F/O AND I WERE PLANNING AND BRIEFING THE LNDG TECHNIQUE TO BE USED SINCE THE WINDS WERE VERY STRONG XWINDS WITH RPTS OF LOW LEVEL WINDSHEAR ON FINAL (500'). CONTINUING OUR APCH STRAIGHT IN AND COMING THROUGH 500' AGL; SMA X XED OUR FLT PATH FROM A LOW 1 O'CLOCK TO AN 11 O'CLOCK POS AT ABOUT 300' ALT. HE AT FIRST CLBED; WE SPOTTED HIM AND DIVERTED TO THE RIGHT; AT WHICH TIME HE SAW US AND STARTED HIS EVASIVE ACTION (A DSCNT) AND PASSED OFF TO OUR LEFT 11 O'CLOCK POS (200' MISS). I PROCEEDED TO RPT THE MISS OVER OUR PRESENT FREQ AND THEN WAS TOLD TO CONTACT TWR. WE WERE NOW AT 200' AGL; 1/2 MI FROM RWY 34R THRESHOLD. CONTACTED TWR AND RECEIVED CLRNC FOR LNDG PRIOR TO T/D. ADVISED GND CTL OF THE INCIDENT. FACTORS: ACFT WITH SIMILAR CALLSIGNS; FREQ CONGESTION; NO ATC HDOF; HEAVY XWINDS--APCH BRIEFING PLANNING; CREW SHOULD HAVE INITIATED TWR HDOF; IF WE HAD BEEN ON TWR FREQ WE MIGHT HAVE HAD THE TFC RPT; TWR DID NOT KNOW OF THE SMA X; SMA X WAS NOT IN COMS WITH TWR; AND AG FIELDS SURROUND ARPT.