all chicken wire will do is prevent the snake from leaving the pen with a belly full of chicks! I know it is alot more expensive but it will last forever and will save you a lot of trouble. You don't have to make your entire coop and runs snake proof. Snakes generally don't mess with adult...
Most chicken snakes here in Texas are 5'-6' so that would not even slow them down! I have 2 grow out pens that are 8X8 and they are hardware cloth from top to bottom.
Sounds like the work of a chicken snake! All of my brooders and grow out pens are mad of 1/2X1/2 hardware cloth for this reason. I tried grown A&M quail in a pen that was 1X2 and the snakes still got them.