Low-temperature (T50 mK) transport characteristics are measured for parallel wires coupled by a ballistic window. The structure is fabricated by focused Ga ion-beam scanning and subsequent annealing. The size of the structure is modified by in-plane-gated operation. At zero magnetic field, the transport characteristics are governed by the mode matching between one-dimensional modes in the wires and quasi-zero-dimensional modes in the window. When four-terminal resistance is measured as a function of in-plane-gate voltage, a small period oscillation is superimposed on the background oscillation corresponding to the subband population in each wire. At intermediate magnetic fields, Aharonov-Bohm interference effects are observed in both the magnetic field and the in-plane-gate voltage dependence. These originate from the circulating channel in the window. Theoretical calculations essentially reproduce the experimental results, supporting these explanations.