  General expressions are derived for the electrical resisitivity and thermal
conductivity of a twinned single crystal. Particular attention is paid to the
effect of the structure of the twin domains on these transport coefficients.
Edge effects are also considered. The expression for the thermal conductivity
is used to fit data for a twinned single crystal of 0.8% Zn-doped YBa2Cu3O6.98.
The expression for the electrical resistivity is used to fit previously
published electrical resistivity data for a twinned single crystal of
YBa2Cu3O6.9. It is found that twin boundaries are not a significant source of
electron scattering in high-quality single crystals of Y-123. We cannot rule
out scattering of phonons by twin boundaries in these crystals, with up to 12%
suppression of the phonon component of the thermal conductivity. The related
problem of determining the electrical resistivity and thermal conductivity of a
crystal with oblique and alternating isotropic regions of different
conductivities is also solved.
