The effects of the geometry and temperature on the quantum conductance for one-dimensional (1D) ballistic transport through a constriction in a 2D electron gas are investigated by use of a refined formalism. As the length of the constriction increases, weak oscillations around the classical conductance evolve into a steplike structure and the resonances on the plateaus become pronounced. Quantization at integer multiples of 2${e}^{2}$/h occurs only for uniform constriction of finite length. At finite temperature of \ensuremath{\sim}0.6 K significantly long uniform constriction is necessary to observe plateaus devoid of resonance structure.