Magnetic-Field-Induced Carrier Conversion in a Charge-Density-Wave Conductor

We have performed narrow-band noise and dc conductivity measurements on the charge-density-wave (CDW) conductor Nb${\mathrm{Se}}_3$ in magnetic fields up to 75 kG. In the lower CDW state, the anomalous magnetoresistance is shown to result from a direct magnetic-field-induced conversion of normal to CDW carriers. The $H$ and $T$ dependence of the new CDW carrier concentration, and hence CDW order parameter, is explicitly determined.