  We investigate the case of phase-matched high-harmonic generation in a
gas-filled capillary waveguide, comparing in detail theory with experiment. We
observe three different regimes of phase matching: one where atomic dispersion
balances waveguide dispersion, another corresponding to non-collinear Cerenkov
phase-matching, and a third where atomic dispersion and plasma dispersion
balance. The role of atomic dispersion is demonstrated by studying the
dependence of the harmonic signal for several gases. We also predict and
provide preliminary evidence of a regime where phase-matching occurs only at
specific fractional ionization levels, leading to an output signal that is
sensitive to the absolute phase of the carrier wave.
