  Theories of microwave-induced resistance oscillations in high-mobility
two-dimensional electron gas predict that with decreasing oscillation order $n$
or with increasing frequency $\omega$ the photoresistance maxima should appear
closer to the cyclotron resonance harmonics due to increased Landau level
separation. In this experimental study we demonstrate that while for a given
$\omega$ the peaks do move towards the harmonics with decreasing $n$, there is
no corresponding movement with increasing $\omega$ for a given $n$. These
findings show that the positions of the photoresistance maxima cannot be
directly linked to the Landau level separation challenging our current
understanding of the phenomenon.
