We report on the observation of quantized conductance up to 10 K in epitaxially regrown, heterostructurally defined, 100-nm-wideGa0.25In0.75As/InPquantum wires. In addition to the plateaus at integer steps of2e2/h,we observe plateaus at0.2(2e2/h),0.7(2e2/h),and1.5(2e2/h),indicating spin polarization at zero magnetic field. Of these, the first two plateaus appear to evolve into one at around0.5(2e2/h)when the sample is subjected to a magnetic field parallel to the quantum wire. The observation of quantized conductance is made possible by the substantial improvement in the quality of the interface by regrowth. ©1997 American Institute of Physics.