[R] Not to draw the xaxis ticks in ggplot2

[ronggui]

>library(ggplot2)
>(p<- qplot(mpg, wt, data=mtcars))
What I am doing is to set color of the ticks to hide them.
>grid.gedit(gPath("xaxis", "ticks"), gp=gpar(col="white"))

It should be a better way to achieve the purpose. Thanks.