  We have investigated in frame of the quantum transport theory the magnetic
quantum oscillations of the longitudinal magnetoresistance $\rho_{zz}$ in quasi
two-dimensional metals for a magnetic field perpendicular to the layers.
  Giant Shubnikov-de Haas oscillations are found when the cyclotron energy
$\hbar \omega_{c}$ is much larger than the interlayer transfer integral $t$
(the two-dimensional limit). In large magnetic fields and at low temperatures,
the minima of the magnetoconductivity $\sigma_{zz}=\rho_{zz}^{-1}$ exhibit a
thermally activated behavior in presence of negligibly small chemical potential
oscillations, as observed in the organic layered conductor
$\beta''\mathrm{-(BEDT-TTF)}_{2}\mathrm{SF}_{5}\mathrm{CH}_{2}\mathrm{CF}_{2}\m
athrm{SO}_{3}$.
  The questions concerning the absence of strong chemical potential
oscillations in such compound and the impurity self-energy are discussed.
