First-principles calculation methods have made a substantial contribution to unveiling the nature of various physical and chemical phenomena observed in nanometer-scale structures. In this article, first-principles calculation methods are applied to the ballistic electron transport problem, which is a typical nanoscale phenomenon and should be taken into account in nanoscale functional systems to be developed in the near future. Our electron transport calculations for single-row atomic nanowires highlight two interesting electron transport properties: one is oscillatory behavior in conductance as a function of the nanowire length, and the other is negative differential conductance. By analyzing these anomalous behaviors in conductance in detail using simplified jellium model, we reveal the origin of the conductance oscillation and the mechanism of negative differential conductance.