  We study a two-dimensional motion of a charged particle in a weak random
potential and a perpendicular magnetic field. The correlation length of the
potential is assumed to be much larger than the de Broglie wavelength. Under
such conditions, the motion on not too large length scales is described by
classical equations of motion. We show that the phase-space averaged diffusion
coefficient is given by Drude-Lorentz formula only at magnetic fields $B$
smaller than certain value $B_c$. At larger fields, the chaotic motion is
suppressed and the diffusion coefficient becomes exponentially small. In
addition, we calculate the quantum-mechanical localization length as a function
of $B$ in the minima of $\sigma_{xx}$. At $B < B_c$ it is exponentially large
but decreases with increasing $B$. At $B > B_c$, the localization length drops
precipitously, and ceases to be exponentially large at a field $B_\ast$, which
is only slightly above $B_c$. Implications for the crossover from the
Shubnikov-de Haas oscillations to the quantum Hall effect are discussed.
