Moving a subscript in {xy-pix}

To obtain a larger version of \nrightarrow (LaTeX, Package amssymb), I typed

$\xymatrix{A\ar@<-1.5ex>@{}[r]^{/}\ar[r]&B}$ 

Is there a simpler way to obtain the same result, i.e. is it possible to move upward or downward the subscript (or the superscript) of an arrow?