SMT JET SLIDES OFF TXWY AFTER CLRING THE RWY THAT HAD POOR TO NIL BRAKING.

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DURING APCH INTO PTK; USING RWY 27L BACK COURSE WE RECEIVED A 'GOOD' BRAKING RPT FROM TWR GIVEN BY A HAWKER 20 MINS EARLIER. LIGHT SNOW HAD BEEN RPTED IN FORECASTS AND CURRENT WX. THERE WAS A LIGHT COATING OF APPROX 1/4 - 1/2 INCH ON RWY. WE BROKE OUT AT 300 FT ABOVE MINIMUMS AND FOLLOWED THE VASI TO THE RWY WITH A SLIGHT FLARE BEFORE TOUCHDOWN. INITIALLY I FELT NORMAL DECELERATION AND I MADE AN 80 KT CALL. FROM THERE THE DECELERATION RATE DIMINISHED. ANTI-SKID WAS FUNCTIONING. I ANNOUNCED I WOULD TAKE CTL OF THE ACFT FROM THE R SEAT WITH HOPES OF A DIFFERENT BRAKING RESPONSE. POOR TO NIL BRAKING CONTINUED. AT 10-15 KTS NEAR THE END OF THE RWY I WAS ABLE TO STEER AND HAD SHUT DOWN THE R ENG. I MADE THE LAST TURNOFF BUT THE MOMENTUM AND NIL BRAKING WAS SUCH THAT WHEN OFF THE RWY WE SLID INTO THE GRASS AND MUD. NO DAMAGE WAS VISIBLE FROM MECH'S INSPECTION AND PAX WAS FINE. WE HAD BRIEFED CONTAMINATED RWY PROCS; BUT IN THIS SIT AGAIN I WOULD ASK FOR A MORE CURRENT BRAKING RPT BY A VEHICLE (IF AVAILABLE); WAIT FOR RWY TO BE SANDED. AFTER WE LANDED THE RWY HAD BEEN PARTIALLY SANDED; AND A FALCON 20 RPTED POOR BRAKING.