  We show that the statistics of electron transfer in a coherent quantum point
contact driven by an arbitrary time-dependent voltage is composed of elementary
events of two kinds: unidirectional one-electron transfers determining the
average current and bidirectional two-electron processes contributing to the
noise only. This result pertains at vanishing temperature while the extended
Keldysh-Green's function formalism in use also enables the systematic
calculation of the higher-order current correlators at finite temperatures.
