Quantum Hall transitions in mesoscopic devices

System-size dependence of the quantum Hall plateau-to-insulator transition in GaAs two-dimensional electron systems is studied, where the system size is comparable to the correlation length of potential fluctuation. The transition exhibited a nonstandard system-size dependence at low temperatures, which is interpreted by assuming that the underlying mechanism of localization crosses over from Anderson localization to semiclassical percolation in mesoscopic systems.