Gauge-invariant and current-continuous microscopic ac quantum transport theory JianQiao Zhang, ZhenYu Yin, Xiao Zheng, ChiYung Yam and GuanHua Chen Eur. Phys. J. B, 86 10 (2013) 423 Published online: 09 October 2013 DOI: 10.1140/epjb/e2013-40325-7