  We calculate the Landau levels of a Kramers-Weyl semimetal thin slab in a
perpendicular magnetic field $B$. The coupling of Fermi arcs on opposite
surfaces broadens the Landau levels with a band width that oscillates
periodically in $1/B$. We interpret the spectrum in terms of a one-dimensional
superlattice induced by magnetic breakdown at Weyl points. The band width
oscillations may be observed as $1/B$-periodic magnetoconductance oscillations,
at weaker fields and higher temperatures than the Shubnikov-de Haas
oscillations due to Landau level quantization. No such spectrum appears in a
generic Weyl semimetal, the Kramers degeneracy at time-reversally invariant
momenta is essential.
