Can electron confinement barriers be determined by STM?

Electrons confined in Pb islands grown on Cu(111) display quantized, particle-in-a-box energy levels that can be detected by scanning tunneling spectroscopy. The evolution of the quantum well states with the thickness of the wells can be fitted by a simple model that positions the known finite confinement barriers slightly underneath the apparent surface. The effect is traced to the spill-over of electrons at surfaces and has to be considered to determine electron barriers by STM.