Nonlinear electron dynamics in a coherent wave packet is investigated via Fokker-Planck equations. Electrons are trapped (and detrapped) by the coherent wave potential whose amplitude is large so that the electron bounce time in the potential well is smaller than the electron transit time through the packet. This effect is now included into the wave-induced friction and diffusion terms in the Fokker-Planck equation. The quasi-steady state distribution of the electrons is obtained to have a tail which is neither a quasilinear one nor a plateau. The wave induced steady current and the ratio of the current to the dissipated power are numerically calculated to be compared to those obtained from the quasilinear theory.