  We investigate the threshold behavior of transmission resonances and
quasibound states in the multichannel scattering problems of a one dimensional
(1D) time-dependent impurity potential, and the related problem of a single
impurity in a quasi 1D wire. It was claimed before in the literature that a
quasibound state disappears when a transmission zero collides with the subband
boundary. However, the transmission line shape, the Friedel sum rule, and the
delay time show that the quasibound states still survive and affect the
physical quantities. We discuss the relation between threshold behavior of
transmission resonances, and quasibound states and their boundary conditions in
the general context of multichannel scatterings.
