We have measured the conductance quantization of a single, ungated and quasi-ballistic GaAlAs/GaAs heterojunction wire using the time decay of the photoconductivity. At temperatures 1.7-4.2 K we observed well resolved conductance plateaux with step heights ΔG ≈ 0.25 × (2e2 h). For T = 0.35 K the conductance quantization is obscured by fully developed universa conductance fluctuations. An increase of the step height to 2e2 h with magnetic field is also observed. The data are compared with existing theories of transport in weakly disordered quasi-one-dimensional wires. © 1992.