Quantum magnetotransport in a nondegenerate two-dimensional electron gas under extremely strong magnetic fields

The quantum magnetotransport properties of a nondegenerate two-dimensional gas of electrons interacting with helium-vapor atoms above a liquid-helium surface is studied in magnetic fields up to 20 T by the ac capacitive coupling technique. The data and the theoretical analysis performed show that in the ultraquantum limit the generalized or effective collision frequency [nu][sub eff] of the electrons increases faster with magnetic field than the cyclotron frequency [omega][sub [ital c]]. Under extremely strong magnetic fields, where the Landau level width becomes comparable to or larger than the thermal energy, the high-cyclotron-frequency approximation [omega][sub [ital c]][much gt][nu][sub eff], usually assumed in quantum transport theories, is no longer valid. The self-consistent Born-approximation theory is extended to be valid for any ratio of [omega][sub [ital c]] to [nu][sub eff]. Then it describes the data perfectly without any adjustable parameter. The results reported here also give strong support to the universality of the linear Hall resistivity.