  Collective coordinate quantization of Dirichlet branes is discussed.
Utilizing Polchinski's combinatoric rule, semiclassical D-brane wave functional
is given in proper-time formalism. D-brane equation of motion is then
identified with renormalization group equation of defining Dirichlet open
string theory. Quantum mechanical size of macroscopically charged D-brane is
illustrated and striking difference from ordinary field theory BPS particle is
emphasized.
