The conductance of a two-dimensional electron gas at the transition from onequantum Hall plateau to the next has sample-specific fluctuations as a functionof magnetic field and Fermi energy. Here we identify a universal feature ofthese mesoscopic fluctuations in a Corbino geometry: The amplitude of themagnetoconductance oscillations has an e^2/h resonance in the transitionregion, signaling a change in the topological quantum number of the insulatingbulk. This resonance provides a signed scaling variable for the criticalexponent of the phase transition (distinct from existing positive definitescaling variables).
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