Figure 1:
(a) Energy spectrum of a clean integer quantum Hall system as a function of guiding center position X. States near the edges disperse and carry current. By filling them to different Fermi energies E_{F}^{(L)} and E_{F}^{(R)}, a net current flows down the sample which yields a quantized Hall resistance R_{H}. (b) Interpretation in terms of classical orbits: In bulk, electrons orbit around a fixed position and do not carry current. At the edges specular reflection induces current-carrying motion.