require("ggplot2")

df <- data.frame(
  x = rep(1:2, each = 50),
  y = rt(100, 3)
)

# However, this approach could not change the horizontal lines width...
ggplot(df, aes(x = factor(x), y = y)) +
  stat_boxplot(geom = "errorbar", stat_params = list(width = 0.5), geom_params = list()) +
  geom_boxplot()