Split-gate ballistic constrictions have been fabricated on two separate InAs/AlSb quantum well heterostructures. Constrictions in the initial material layer structure displayed quantized conductance to temperatures as high as 30 K, but suffered from poor reliability and large gate leakage current. Self-consistent Schrodinger-Poisson calculations indicate that donorlike states at the InAs-AlSb interface may aggravate the problem of gate leakage. Constrictions fabricated on an improved layer structure had gate leakage currents several orders of magnitude lower than in the original layer structure. This allowed measurement of quantized conductance with a dc measurement, up to bias voltages of 8 mV.