  A breakdown of integer quantum Hall effect (IQHE) at strong disorder is
studied numerically in a lattice model. We find a generic sequence by which the
integer quantum Hall plateaus disappear: higher IQHE plateaus always vanish
earlier than lower ones. We show that extended levels between these plateaus do
not float up in energy but keep merging together after the disappearance of
plateaus, which eventually leads to a localization in the whole system. We also
study this phenomenon in terms of topological properties, which provides a
simple physical explanation.
