Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We have studied the effect of thermal effects on the structural and transport response of Ag atomic-size nanowires (NWs) generated by mechanical elongation. Our study involves both time-resolved atomic resolution transmission electron microscopy imaging and quantum conductance measurement using an ultra-high-vacuum mechanically controllable break junction. We have observed drastic changes in conductance and structural properties of Ag nanowires generated at different temperatures (150 and 300 K). By combining electron microscopy images, electronic transport measurements, and quantum transport calculations, we have been able to obtain a consistent correlation between the conductance and structural properties of Ag NWs. In particular, our study has revealed the formation of metastable rectangular rod-like Ag wire (3/3) along the [001] crystallographic direction, whose formation is enhanced. These results illustrate the high complexity of analyzing structural and quantum conductance behaviour of metal atomic-size wires; also, they reveal that it is extremely difficult to compare NW conductance experiments performed at different temperatures due to the fundamental modifications of the mechanical behavior. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729805]11112Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)LNLSFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)