  We studied the thermal conductivity of a Bi_2Sr_2CaCu_2O_8 single crystal as
a function of a magnetic field ramped up and down and then reversed. As
recently discovered by Krishana et al., we observed a field-independent thermal
conductivity above a threshold field. However, our measurements show that the
magnitude of background thermal conductivity depends on the field profile in
the sample. This is incompatible with the interpretation put forward by
Krishana and co-workers.
