I'm a little afraid of asking this for I suspect the answer is obvious but...
In my copy of Knight Physics, it states that the "average distance an electron travels between collisions" in air is 2.0 microns. It then uses this to determine the breakdown field strength.
When I look up the mean free path in air I get 69.1 nm (from Handbook of Chemistry and Physics).
I can sort of understand why the electron will travel further then air molecules before colliding but why such a big difference? Is it just the size that makes the difference and/or is it that the electrons travel parallel to the field, not randomly?