Theoretical approaches to electron field emission from nanostructured cathodes often need to predict stationary vacuum currents by means of the time-dependent decay theory of metastable states. This rigorous (but practically untractable) treatment is usually mitigated by various procedures. In this work the authors present a new method based on the hypothesis of the continuity of the electron localization probability at the vacuum interface of the heterostructure. The method is compared to other conventional approaches, in terms of both the obtained vacuum probability current and field-emission current. The computed probability current is very close to that obtained from conventional approaches for the same energy spectra. However, conventional methods fail to predict the field-emission behavior from shallow-well heterostructures. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3100657]