QUANTUM MAGNETOTRANSPORT IN A NONDEGENERATE 2-DIMENSIONAL ELECTRON-GAS UNDER EXTREMELY STRONG MAGNETIC-FIELDS

The quantum magnetotransport properties of a nondegenerate two-dimensi onal gas of electrons interacting with helium-vapor atoms above a liqu id-helium surface is studied in magnetic fields up to 20 T by the ac c apacitive coupling technique. The data and the theoretical analysis pe rformed show that in the ultraquantum limit the generalized or effecti ve collision frequency nu(eff) of the electrons increases faster with magnetic field than the cyclotron frequency omega(c). Under extremely strong magnetic fields, where the Landau level width becomes comparabl e to or larger than the thermal energy, the high-cyclotron-frequency a pproximation omega(c)>>nu(eff), usually assumed in quantum transport t heories, is no longer valid. The self-consistent Born-approximation th eory is extended to be valid for any ratio of omega(c) to nu(eff). The n it describes the data perfectly without any adjustable parameter. Th e results reported here also give strong support to the universality o f the linear Hall resistivity.