Opposite---it's been disproven: http://www.freerepublic.com/focus/news/824600/posts.
a) There's some chemical in marijuana which causes the smoker to go on to harder illegal drugs,
b) Marijuana smokers become dissatisfied with the "high" and desire a more powerful illegal drug,
c) Illegal drug users, generally speaking, start with less powerful drugs like marijuana then work their way up,
d) With some people, smoking marijuana leads to harder illegal drug usage, but we don't know why,
e) Smoking marijuana may lead to harder illegal drugs, but only if combined with other, as yet unidentified, factors,
f) Users of illegal hard drugs like those drugs. Marijuana was a step but not a factor,
f) No drug 'leads to' another drug, period.