  We calculate the dc conductivity $\sigma$ in a pseudogapped high $T_c$
superconductor within a theory which is consistent with gauge invariance. Our
results contain additional terms beyond those identified previously. Although
it has been thought that lifetime effects dominate the $T$ dependence of
transport, here we show (consistent with growing experimental support) that the
temperature dependence of the effective carrier number $(n/m(T))_{\rm{eff}}$
plays a critical role and thereby leads to the contrasting behavior between
over and under-doped regimes. The role of Fermi arcs is secondary.
