Quantum Transport in the Charge-Density-Wave State of the Quasi Two-Dimensional Bronzes (${\bf PO_2}$)$_{\bf 4}$(${\bf WO_3}$)$_{\bf 2m}({\bf m}={\bf 4, 6})$

Magnetotransport has been studied on the quasi two-dimensional monophosphate tungsten bronzes (PO$_2$)$_4$(WO$_3$)$_{2m}$ for $m=4$ and 6, between 0.3 and 300 K in fields up to 18 T. These compounds show several charge density wave transitions. Large magnetoresistance is found in the charge-density-wave state for magnetic fields applied perpendicular to the layers. At low temperatures, Shubnikov-de Haas oscillations are attributed to the existence of small carrier pockets left by the charge density wave gap opening. The size of these pockets is of the order of a few % of the two-dimensional high temperature Brillouin zone and smaller in the case $m=6$ than in $m=4$. This is due to a more pronounced low-dimensional character and therefore to a better Fermi surface nesting in the compound $m=6$ than in $m=4$