  We study the Landau level spectrum using a multi-band
$\mathbf{k}\cdot\mathbf{p}$ theory in monolayer transition metal dichalcogenide
semiconductors. We find that in a wide magnetic field range the Landau levels
can be characterized by a harmonic oscillator spectrum and a linear-in-magnetic
field term which describes the valley degeneracy breaking. The effect of the
non-parabolicity of the band-dispersion on the Landau level spectrum is also
discussed. Motivated by recent magnetotransport experiments, we use the
self-consistent Born approximation and the Kubo formalism to calculate the
Shubnikov de-Haas oscillations of the longitudinal conductivity. We investigate
how the doping level, the spin-splitting of the bands and the broken valley
degeneracy of the Landau levels affect the magnetoconductance oscillations.
Motivated by recent experiments we consider monolayer MoS$_2$ and WSe$_2$ as
concrete examples and compare the results of numerical calculations and an
analytical formula which is valid in the semiclassical regime. Finally, we
briefly analyze the recent experimental results (Reference [18]) using the
theoretical approach we have developed.
