Using the Landauer-Büttiker formalism, we calculate the effect of structural twist on electron transport in conducting carbon nanotubes. We demonstrate that even a localized region of twist scatters the propagating π electrons and induces the opening of a (pseudo-) gap near the Fermi level. The subsequent conductance reduction may be compensated by an applied axial magnetic field, leading to a twist-induced, giant positive magneto-conductance in clean armchair nanotubes.