  Four regimes of vortex motion were identified in the magnetoresistance of
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals: (1) thermally activated
flux flow (TAFF) in samples with surface defects caused by thermal annealing;
(2) TAFF-like plastic motion of highly entangled vortex liquid at low
temperatures, with $U_{pl} \sim (1-T/T_c)/H^{1/2}$; (3) pure free flux flow
above the region of (2) in clean and optimally doped samples; or, in its place,
(4) a combination of (2) and (3). This analysis gives an overall picture of
flux motion in Bi cuprates.
