  The disappearance of integer quantum Hall effect (IQHE) at strong disorder
and weak magnetic field is studied in a lattice model. A generic sequence by
which the IQHE plateaus disappear is revealed: higher IQHE plateaus always
vanish earlier than lower ones, and extended levels between those plateaus do
not float up in energy but keep merging together after the destruction of
plateaus. All of these features remain to be true in the weak-field limit as
shown by the thermodynamic-localization-length calculation. Topological
characterization in terms of Chern integers provides a simple physical
explanation and suggests a qualitative difference between the lattice and
continuum models.
