CC Model: Saddle points in the random potential landscape scatter electrons between Landau level drifting states. Transmission t through each saddle is tuned by energy.
Critical: At the transition (E=0), ξ(E)∝|E|^{-ν} with ν≈2.3 (numerically). This is a 2D quantum percolation problem.
Plateaux: σ_xy=ne²/h on plateaux, transitions happen only at critical energies.