Magnetoresistance of high-Tcsuperconductors in the fluctuation regime

We study theoretically the fluctuation-induced electric conductivity in a magnetic field in the clean limit. Then we analyze the magnetoresistance observed in single-crystal Y-Ba-Cu-O samples. The present theory describes the observed magnetoresistance as well as the earlier dirty-limit theory by Aronov et al. [Phys. Rev. Lett. 62, 965 (1989)] but the value of the phase-relaxation time ${\mathrm{\tau }}_{\mathrm{\phi }}$ is two times larger than the one deduced within the dirty-limit theory.