Quantized conductance measurements on InAs/AlSb split-gate ballistic constrictions with a varying perpendicular magnetic field are reported. The magnetic depopulation of the one-dimensional subbands is used to determine the energy spacing of the subbands in the constriction. Assuming a parabolic lateral-confining potential, a theoretical fit to our data reveals subband spacings as large as 10 meV. For fields greater than ∼2 T, conductance plateaus emerge at odd integer multiples of e2/h, indicating the lifting of the spin degeneracy. The magnetic-field dependence of the G=3e2/h conductance step width is used to obtain a rough estimate of the effective g factor in the constriction.