  The role of quantum tunneling effect in the electron accretion current onto a
negatively charged grain immersed in isotropic plasma is analyzed, within the
quasiclassic approximation, for different plasma electron distribution
functions, plasma parameters, and grain sizes. It is shown that this
contribution can be small (negligible) for relatively large (micron-sized) dust
grains in plasmas with electron temperatures of the order of a few eV, but
becomes important for nano-sized dust grains (tens to hundreds nm in diameter)
in cold and ultracold plasmas (electron temperatures ~ tens to hundreds of
Kelvin), especially in plasmas with depleted high-energy "tails" in the
electron energy distribution.
