REFLECTION OF BALLISTIC ELECTRONS BY DIFFUSIVE 2-DIMENSIONAL CONTACTS

We report quenching of high-magnetic-field Shubnikov-de Haas oscillati ons and shrinking of the Hall plateaux with decreasing temperature in a quantum Hall effect conductor with low-mobility contact leads. Selec tive He-ion bombardment has been employed to reduce the mobility of a two-dimensional electron gas in the contacts. We explain the results i n terms of the Milne reflection of the electrons from these disordered regions. At sufficiently low temperatures, inelastic scattering is su ppressed and electrons are scattered back by elastic collisions before thermalisation can occur.