We investigate the electronic transport characteristics of a one-dimensional (1D) narrow constriction defined in a GaAs/AlxGa 1-xAs heterostructure by a simple triple-gate structure consisting of a pair of split gates and an additional surface Schottky gate (center gate) between them. Comparison between devices with and without a center gate reveals that the center gate, even when zero biased (VCG=0 V), significantly modifies the surface potential and facilitates the 1D confinement in a deep two-dimensional electron system. The pinch-off voltages at VCG=0 V for various channel widths W (=0.4-0.8 μm) and lengths L (=0.2-2 μm) are well described by the analytical formula based on the pinned-surface model [J. H. Davies et al., J. Appl. Phys. 77, 4504 (1995)]. Nonlinear transport spectroscopy with an additional dc bias shows that the lowest 1D subband energy separation (ΔE1,2) changes linearly with VCG and can be enhanced by 70% for VCG=0.8 V. A simple model assuming an infinitely long channel and no self-consistent potential well reproduces the overall behavior of the measured ΔE1,2. In addition, effects of impurities, occasionally found for long-channel devices (L ≥ 1 μm), are found to be greatly reduced by applying positive VCG and thereby enhancing ΔE1,2. Data are also presented for the transport anomaly below the first conductance plateau, the so-called "0.7 anomaly," demonstrating that the triple-gate structure is useful for the study of density-dependent phenomena in a 1D system.