  The nucleon momentum distribution $n_A(k)$ for $A=$2, 3, 4, 16, and 40 nuclei
is systematically analyzed in terms of wave functions resulting from advanced
solutions of the nonrelativistic Schr\"{o}dinger equation, obtained within
different many-body approaches. Particular attention is paid to the separation
of the momentum distributions into the mean-field and short-range correlations
(SRC) contributions. It is shown that at high values of the momentum $k$ the
high-momentum components ($k\gtrsim 1.5-2$ fm$^{-1}$) of all nuclei considered
are very similar, exhibiting the well-known scaling behavior with the mass
number $A$, independently of the used many-body approach and the details of the
bare $NN$ interaction. The number of $NN$ pairs in a given ($ST$) state, viz.,
($ST$)=(10), (00), (01), and (11), and the contribution of these states to the
nucleon momentum distributions are calculated. It is shown that, apart from the
(00) state, which has very small effects, all other spin-isospin states
contribute to the momentum distribution in a wide range of momenta. It is shown
that that for all nuclei considered the momentum distributions in the states
T=0 and T=1 exhibit at $k\gtrsim 1.5-2$ fm$^{-1}$ very similar behaviors, which
represents strong evidence of the A-independent character of SRCs. The ratio
$n_A(k)/n_D(k)$ is analyzed in detail stressing that in the SRC region it
always increases with the momentum and the origin of such an increase is
discussed and elucidated. The relationships between the one- and two-body
momentum distributions, considered in a previous paper, are discussed and
clarified, pointing out the relevant role played by the center-of-mass motion
of a correlated pair in the (10) state. The relationship of the present
approach with the many-body methods based upon low-momentum effective
interactions is briefly discussed.
