Incoherent versus coherent interlayer transport in layered conductors under a magnetic field

Comparative studies of the interlayer magnetoresistance of the layered organic conductor $\alpha$-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$ as a function of the magnetic field orientation have been performed on different samples. The results obtained on the high quality crystal reveal a well defined three-dimensional Fermi surface and provide an evaluation of the interlayer transfer integral, $t_{\bot }\cong 0.04$ meV. The lower quality crystal exhibits a very different behavior which is most likely caused by the breakdown of the three-dimensional coherence, when the scattering rate $\tau ^{-1}$ exceeds the interlayer hopping rate $\sim t_{\bot }/\hbar $