 On the Movement of Small Particles Suspended in a Stationary Liquid Demanded by the Molecular-Kinetic Theory of Heat by Albert Einstein

Primary paper, in the English translation collected in Investigations on the Theory of the Brownian Movement: sites.pitt.edu/~jdnorton/lectures/Rotman_Summer_School_2013/Einstein_1905_docs/Einstein_Brownian_English.pdf

Sections 3–5 connect equilibrium drift and diffusion, the spherical-particle drag coefficient, and mean-squared displacement. The linked scan includes the 1926 translation reprinted in the 1956 Dover edition; the research paper itself is dated 1905.