Relaxation of high-energy quasiparticle distributions: Electron-electron scattering in a two-dimensional electron gas

A theory is developed for the evolution of the nonequilibrium distribution of quasiparticles when the scattering rate decreases due to particle collisions. We propose a ``modified one-collision approximation,'' which is most effective for high-energy quasiparticle distributions. This method is used to explain measurements of the nonmonotonic energy dependence of the signal of scattered electrons in a two-dimensional system. The observed effect is related to a crossover from the ballistic to the hydrodynamic regime of electron flow.