1970s; earliest use found in Philosophical Transactions of the Royal Society of London. From mesoscopic + -ally, after microscopically, macroscopically.
Definition of mesoscopically in US English:
mesoscopically
adverbˌmɛsə(ʊ)ˈskɒpɪkli
On a mesoscopic scale.
Origin
1970s; earliest use found in Philosophical Transactions of the Royal Society of London. From mesoscopic + -ally, after microscopically, macroscopically.