POTENTIAL GND CONFLICT WITH TAXIING ARR LNDG TFC. UNAUTH RWY OP ENTRY XING.

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I HAD BEEN CLRED TO LAND ON RWY 18; FCM. THE FIELD HAS A N SIDE TXWY WHICH CROSSES 18 A FEW FT FROM THE THRESHOLD. RUN-UP AREAS ARE ON EITHER SIDE OF THE RWY AND OCCUPY SPACE BTWN THE TXWY AND THE N END OF THE RWY. AS I WAS APPROX 100 FT FROM THE END OF THE RWY; I NOTICED 1 OF THE ACFT WHICH I HAD INITIALLY THOUGHT TO BE IN THE RUN-UP AREA WAS PROCEEDING ACROSS THE RWY; W TO E. I RAISED THE R WING AND PULLED BACK ON THE YOKE ENOUGH TO ACTIVATE THE STALL WARNING AND THEN PUSHED THE NOSE DOWN MAKING AN UNEVENTFUL LNDG; EXCEPT THAT I WAS L OF THE CTRLINE. THE TWR CONTACTED ME DURING THE ROLLOUT AND ADVISED THAT THE ACFT; WHICH HAD CROSSED THE RWY DID SO WITHOUT PERMISSION. THERE ARE SEPARATE GND AND TWR FREQS SO I WAS UNAWARE OF ANY DIRECTIONS BEING GIVEN TO THE OTHER ACFT. THE ACFT I WAS FLYING DID NOT HAVE A MICROPHONE SWITCH ON THE YOKE. I WAS USING A HEADSET WITH THE XMIT SWITCH IN MY LAP. ALTHOUGH THE XMIT SWITCH CAN BE SECURED TO THE YOKE WITH VELCRO; I BELIEVE IT INTERFERES WITH PROPER CTL OF THE ACFT. HAD THERE BEEN A YOKE-MOUNTED MIKE SWITCH; I COULD HAVE CONTACTED THE TWR BEFORE GETTING INTO A POTENTIALLY DANGEROUS SIT. I THOUGHT UNTIL THE VERY LAST MOMENT THAT THE TAXIING ACFT WAS GOING TO OBSERVE FIELD CUSTOM AND STOP. I BELIEVE THE LESSON TO BE LEARNED IS TO INITIATE A GAR FIRST RATHER THAN ANTICIPATING PROPER SAFETY PRECAUTIONS ON THE PART OF THE OTHER PLT.