22. Z. L. Guan, Y. X. Ning, C. L. Song, J. Wang, J. F. Jia, X. Chen, Q. K. Xue, and X. C. Ma, Sample-size dependence of the superconducting transition of ribbon-shaped Pb nanocrystals studied by scanning tunneling spectroscopy, PHYSICAL REVIEW B 81, 054516 (2010).