  The integer quantum Hall effect is analysed using a transport mechanism with
a semi-classic wave packages of electrons in this paper. A strong magnetic
field perpendicular to a slab separates the electron current into two branches
with opposite wave vectors $({\it k})$ and locating at the two edges of the
slab, respectively, along the current. In this case back scattering of
electrons ($k\rightarrow -k$) is prohibited by the separation of electron
currents. Thus the slab exhibits zero longitudinal resistance and plateaus of
Hall resistance. When the Fermi level is scanning over a Landau level when the
magnetic field increases, however, the electron waves locate around the central
axis of the slab and overlap each other thus back scattering of electrons takes
place frequently. Then longitudinal resistance appears and the Hall resistance
goes up from one plateau to a new plateau.
