prevent form authentication cookie does not work

Hi friends,

I dont want brower keeps authentication cookies.

this is my web.config and these code that I write doesn't work, 

<authentication mode="Forms"><forms 
        cookieless="UseUri"
        defaultUrl="index.aspx"
        loginUrl="login.aspx"
        timeout="30"/></authentication>

i get confuse by authentication cookie and session cookie.

which prevents the brower to keep cookie?