Magnetic quantum oscillations in quasi-two-dimensional organic conductors

Bechgaard salts like (TMTSF${)}_{2}$X with X=${\mathrm{PF}}_{6}$, ${\mathrm{NO}}_{3}$, and ${\mathrm{ClO}}_{4}$ exhibit Shubnikov\char21{}de Haas\char21{}like oscillations, which are not described within the standard model. In addition (TMTSF${)}_{2}$${\mathrm{NO}}_{3}$ exhibits slow oscillations with characteristic periods rather similar to those corresponding, to the field-induced spin-density waves found in (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$ and (TMTSF${)}_{2}$${\mathrm{PF}}_{6}$ under pressure. I propose a simple theory which describes these phenomena.