Deviation of the Quantum Hall Effect from Exact Quantization in Narrow GaAs/AlGaAs Heterostructure Devices.

We report deviation for the i=4 quantum Hall plateau from its quantized value, in narrow GaAs-AlGaAs quasi-1-d wires of width ~2000angstroms. The deviation arises in the form of aperiodic fluctuations as the magnetic is varied, even though a deep minimum developes in longitudinal resistance. The fluctuation size grows with decreasing temperature and can be as large as 250omega at 50mK. We suggest the observations arise from a combination of localization and Aharonov-Bohm quantum interference effects.