The Keldysh-Green function technique in the representation of scattering states is used to study the dynamic conductance and the stationary current noise of a mesoscopic tunnel junction. It is shown that, despite the generally accepted viewpoint [M. Buttiker, Phys. Rev. B 45, 3807 (1992)], within the framework of the one-electron approximation the linear ac current response cannot be expressed in terms of the one-electron scattering matrix, but rather it is completely determined by the true electron wave functions in the region of the potential barrier. In addition to high-frequency device applications, the study of dynamic conductance has fundamental importance in providing information about the structure of the potential barrier. We analyze the linear ac current response of electron field emission and show that, if the tunneling electron is affected by the electrostatic image force, the low-frequency field emission stationary current noise spectral density has flicker-noise behavior similar to 1/omega(4/3) over a wide frequency range. Experimental evidence for this frequency dependence has been reported by a number of authors.