Mesoscopic fluctuations in high magnetic fields: Change in behavior due to boundary diffusion

We have studied universal conductance fluctuations in the magnetoresistance of n+-type GaAs submicrometer wires, which represent an orthodox mesoscopic system but also allow us to reach the high-magnetic-field regime, ωcτ>1, where ωc is the cyclotron frequency and τ the electron scattering time. The Lee-Stone correlation field Bc increases by more than an order of magnitude as the magnetic field increases from 0 to 18 T, but the amplitude of the fluctuations remains unchanged. This implies that the universal scaling of conductance fluctuations is not valid in high magnetic fields, in strong disagreement with theoretical predictions. We show that this behavior is not specific to the nonlocal geometry of measurements, where the breakdown has been reported earlier, but that it also occurs in the local magnetoresistance and rectification fluctuations. The violation of universal scaling is attributed to the appearance of a second phase-breaking length, in the regime ωcτ>1, due to extended electron diffusion near the sample boundaries. © 1993 The American Physical Society.