  In a quasi two-dimensional electron system with non-zero layer thickness, a
parallel magnetic field (B||) can couple to the out-of-plane electron motion
and lead to a severe distortion and eventual disintegration of the Fermi
contour. Here we directly and quantitatively probe this evolution through
commensurability and Shubnikov-de Haas measurements on electrons confined to a
40-nm-wide GaAs (001) quantum well. We are able to observe the Fermi contour
disintegration phenomenon, in good agreement with the results of semi-classical
calculations. Experimentally we also observe intriguing features, suggesting
magnetic-breakdown-type behavior when the Fermi contour disintegrates.
