The critical role of the CuO chains in metallizing the c-axis in YBa2Cu3O7-delta

We report measurements of the normal state c-axis magnetoresistance (MR) of untwinned single crystals of slightly overdoped YBa2Cu3O7-delta An orbital contribution to the transverse MR, a signature of coherent c- axis motion, arises only from the component of the magnetic field perpendicular to the CuO chains. This result implies that the CuO chains are primarily responsib le for coherent transport along the c-asis in YBa2Cu3O7-delta and that thos e regions of the CuO2 planes that do not involve the chains remain two-dime nsional, even in the absence of a normal state gap.