This study was prompted by the open question concerning the mechanism and corresponding nonlinearity in traditional Fowler-Nordheim (FN) plots of electron-field emission from advanced nanocarbons. Experimental field emission results from nanostructured carbons, including sulfur-incorporated nanocrystalline diamond as well as carbon nanotubes, are discussed that do not provide traditional linear FN plots. To overcome this difficulty, the authors suggest a multistep phenomenological model that divides the FN plots into high and low regimes of applied macroscopic electrostatic field. The authors describe the difference of FN slopes between high and low field by using a distribution function for energy levels. The authors suggest that the field emission mechanism from nanostructured carbons consists of two successive processes: (1) tunneling of electrons through the low-energy barrier from highly doped semiconducting or metallic region into the semiconducting (metallic) region under the external macroscopic electric field; and (2) tunneling through the high (low) energy barrier from the semiconducting (metallic) region into vacuum under the Coulomb field of an additional electron appearing in the first-step process. The metallic region in step (2) is assigned to the multiwalled nanotubes, which are invariably metallic in nature in contrast to the single-walled nanotubes, which consist of an electrically heterogeneous mixture of semiconducting and metallic types with a finite band gap. (C) 2008 American Vacuum Society.