#JavaScript, <s>128</s> <s>125</s> <s>123</s> 114 bytes

    n=>{r='';for(i=0;i<n;i++)r+='/\\';for(j=0;j<2*n-1;j++){r+='\n'+(j%2?'':'\\');for(i=n-j/2;i>0;i--)r+='/ '}return r}

De-golf (also converted to ES5) + demo:

<!-- begin snippet: js hide: false -->

<!-- language: lang-js -->

    function c(n) {
        r = '';
        for (i = 0; i < n; i++) r += '/\\';
        for (j = 0; j < 2 * n - 1; j++) {
            r += '\n' + (j % 2 ? '' : '\\');
            for (i = n - j / 2; i > 0; i--) r += '/ '
        }
        return r
    }

    alert(c(prompt()));

<!-- end snippet -->