  The single-particle state is not expected to demonstrate second-order
coherence. This proposition, correct in the case of a pure quantum state, is
not verified in the case of a mixed state. Here I analyze the consequences of
this fact for the second-order correlation function, $G ^{(2)}$, of electrons
injected on top of the Fermi sea with nonzero temperature. At zero temperature,
the function $G ^{(2)}$ unambiguously demonstrates whether the injected state
is a single- or a multi-particle state: $G^{(2)}_{}$ vanishes in the former
case, while it does not vanish in the latter case. However, at nonzero
temperatures, when the quantum state of injected electrons is a mixed state,
the purely single-particle contribution makes the function $G ^{(2)}_{}$ to be
non vanishing even in the case of a single-electron injection. The
single-particle contribution puts the lower limit to the second-order
correlation function of electrons injected into conductors at nonzero
temperatures. The existence of a single-particle contribution to $G ^{(2)}_{}$
can be verified experimentally by measuring the cross-correlation electrical
noise.
