A case is made for the existence of permeability of current sheets in electromagnetic pulsed plasma thrusters. Permeability refers to the leakage of plasma or neutral particles behind the sheet and could cause a significant effect on the performance of the thruster. Analytical calculations are presented which demonstrate the mismatch between experimental velocity profiles and theoretical snowplow performances. Electron density measurements show a significant plasma wake following the sheet. Arguments are also presented based on these data, that in some cases the accelerating plasma sheet entrains less than one third of the propellant. An index of permeability is defined as a ratio of sheet-trailing mass to encountered propellant mass. It may be quantified either by directly measuring the trailing mass, or by measuring the mass entrained by the sheet. The diagnostics needed to measure this index are discussed.