  We study phase coherent transport in a single channel system using the
scattering matrix approach. It is shown that identical vanishing of the
transmission amplitude occurs generically in quasi-1D systems if the
time-reversal is a good symmetry. The transmission zeros naturally lead to
abrupt phase changes (without any intrinsic energy scale) and in-phase
resonances, providing insights to recent experiments on phase coherent
transport through a quantum dot.
