We review several models of two dimensional systems exposed to a magnetic field perpendicular to the plane with the emphasis on the transport caused by various erturbations. One concerns a parabolic potential channel; we discuss how is the ac spectrum modified by periodic and "random" perturbations. In other models we have just   singular perturbation supported by an array of points or by a curve. In particular, we show the existence of persistent currents if the latter is a loop and the perturbation is  trong enough.
 

Last updated: 28.08.2002