  The conductance of a two-dimensional electron gas at the transition from one
quantum Hall plateau to the next has sample-specific fluctuations as a function
of magnetic field and Fermi energy. Here we identify a universal feature of
these mesoscopic fluctuations in a Corbino geometry: The amplitude of the
magnetoconductance oscillations has an e^2/h resonance in the transition
region, signaling a change in the topological quantum number of the insulating
bulk. This resonance provides a signed scaling variable for the critical
exponent of the phase transition (distinct from existing positive definite
scaling variables).
