Shielding a PTT line adds unneccesary weight. When you push the button it grounds the line. 22 or even 24 AWG wire is fine. Remember, an ounce here, an ounce there, now you have a pound. Mic and audio lines can be 24 AWG shielded lines. Terminate the shields properly and you should be fine. I use heat shrinkable environmental terminations. Pricey but they will easily last the life of the aircraft.