Coherence effects above Tc in YBa2Cu4O8

High temperature superconductors (HTSC) consist of CuO2 layers which are separated by charge reservoirs (CuO-chains). They show strongly anisotropic physical parameters. Lower doped samples reveal metallic conductivity inside the layers but insulating temperature dependence for the out-of-plane direction. The resistivities of highly doped samples are metallic for all three directions. Evidence is presented that the temperature dependence for the resistivity perpendicular to the CuO2 layers is related to the charge reservoirs in between the planes. The metallic out-of-plane resistivity might be traced back to a metallic conduction channel due to the charge reservoirs.