  We analyze the weak localization correction to the conductivity of a spinless
two-leg ladder model in the limit of strong dephasing \tau_\phi << \tau_tr,
paying particular attention to the presence of a magnetic field, which leads to
an unconventional magnetoresistance behavior. We find that the magnetic field
leads to three different effects: (i) negative magnetoresistance due to the
regular weak localization correction (ii) effective decoupling of the two
chains, leading to positive magnetoresistance and (iii) oscillations in the
magnetoresistance originating from the nature of the low-energy collective
excitations. All three effects can be observed depending on the parameter
range, but it turns out that large magnetic fields always decouple the chains
and thus lead to the curious effect of magnetic field enhanced localization.
