Abstract
Monte Carlo simulations of electron transport have been performed in methane for time resolved rf fields under conditions where negative differential conductivity (NDC) occurs. It was found that the effect of NDC occurs for low frequencies as expected from the quasi stationary (temporally local) model. At higher frequencies the NDC disappears gradually but the NDC behavior is significantly different when field increases and when field decreases. In the latter case the NDC disappears while in the former it blends into one maximum occurring at the point of the maximum of the electric field with an intermediate frequency region with asymmetric time dependence.