  When a quantum wire is weakly confined, a conductance plateau appears at
e^2/h with decreasing carrier density in zero magnetic field accompanied by a
gradual suppression of the 2e^2/h plateau. Applying an in-plane magnetic field
B|| does not alter the value of this quantization; however, the e^2/h plateau
weakens with increasing B|| up to 9 T, and then strengthens on further
increasing B||, which also restores the 2e^2/h plateau. Our results are
consistent with spin-incoherent transport in a one-dimensional wire.
